<!DOCTYPE html>
<html>

<head>
    <title>Example</title>
    <style type="text/css">
        :invalid {
            outline: medium solid red;
        }
        :valid {
            outline: medium solid green;
        }
    </style>
</head>

<body>
    <form action="" method="POST">
        <p>
            <label for="name">Name: <input type="text" required id="" name=""></label>
        </p>
        <p>
            <label for="city">City: <input type="text" id="" name=""></label>
        </p>
        <button type="submit">Submit Vote</button>
    </form>
</body>

</html>